Jose wrote:

[You're established inbound on an NDB] When within 10 degrees of the
published course.

There is no policy or rule to support that.


Taking Sam Spade's TERPs quote,

The primary area is longitudinally centered on the FAC, and is 10
miles long. The primary area is 2.5 miles wide at the facility, and
expands uniformly to 6 miles wide at 10 miles from the facility.


I draw a little diagram, and find that the angle along the outside of
the primary area to be atan(((6/2)-(2.5/2))/10) = atan(7/40) = 10
degrees. So, ten degrees from the facility puts me 2.5 miles inside the
protected area. Sounds good to me. Start on down.

Jose
That is right at the limits without any compass or ADF error. Not a
good idea. But, it's your call